We are actually forbidden to talk to the dead; Saul was reprimanded for calling Elijah up to talk to him.
So it can be done, but we are not supposed to do it.
We ask the souls in heaven and in Purgatory to pray for us. That’s not calling up someone to chit-chat with them. And remember that the soul is alive, even though the body is dead.
The body will be reunited with the soul at the Final Judgment.
Saul got in trouble for (a) visiting a witch; (b) conjuring, via the witch, the ghost of Samuel (not Elijah, who wouldn’t be born for another 400 years); for the purpose of (c) telling Saul’s fortune. So he had three strikes against him, none of which were merely for communicating with Samuel. If any sort of communication with the dead were evil, Jesus set a terribly bad example for us by doing it himself in the presence of his three principle disciples.
....And why is that so?
Actually Saul called Samuel, not Elija.
But yes, he was sternly rebuked for this.
No, the method Saul used was forbidden by God and Jewish Law.
I understand you point. On the other hand petitioning a saint, who is a member of the Church triumphant, requesting his prayers for your behalf, does not seem like the same thing.